Color television



March 16, 1948t P. c. GOLDMARK COLOR TELEVISION Filed Dac. l2, 1945 Pon/er Mams l veraf/d M ATTORNEYS Patented Mer. 16,1948

COLOR TELEVISION Peter C. Goldmark, New Canaan, Conn., asslgnor to Columbia Broadcasting System, Inc., New York, N. Y., a corporation of New York Application December 12, 1945, serial No. 634,464

9 claims. (ci. 17a-5.2)

This invention relates primarily to color television, particularly to color television apparatus employing a moving iilter element. In its broader aspects, however, it can be used in other fields. While especially designed and adapted for .use in color television receivers, it can also be applied to transmitters.

The color television system preferred at the present time is of the sequential type, in which video signals corresponding to different primary colors of an object field are successively generated, transmitted and reproduced. Commonly, successive iield scansions correspond to different primary colors, and interlaced scansion is preferred. Such a system has been described in my application Serial No. 355,840, filed September 7J 1940, and has been found satisfactory in practice.

In apparatus at present employed, a moving lter element, such as a rotating disk, drum or other form, is utilized at the transmitter so that successive portions of the video signal correspond to successive color-separation values of the object field. At the receiver a similar iilter element is employed, usually in cooperation with a cathode-ray receiver tube, to exhibit successive color-separation images in their respective colors. By analogy to the color photographic art, the term color-separation image refers to a monochrome image, usually black and White. representing a primary color. Such an image may bethat of the complete object eld to be reproduced, or only a portion thereof.

For proper operation it is necessary that the rotating filter element move in synchronism and correct'phase with the transmitting or receiving scanning device employed. Referring to the receiver for convenience, it is necessary the iilter element rotate in synchronism with the reproduction of successive color-separation images, so that successive images will be exhibited through successive lters of corresponding color. It is also necessary that the moving lter be phased with respect to the images so that, say, the red colorseparation image will be displayed through the red lter, the green color-separation image through the green lter, etc. This may be termed color phasing. An additional requirement is that the iilter segments be properly phased with respect to the line-by-line reproduction of corresponding images, so that as successive lines are reproduced they will be exhibited through the proper color filter. If there is any afterglow or storage effect in the image reproduction, the iilter segments must be sufficiently Wide and properly 2 phased so that proper color rendition is obtained. This may be termed color field phasing of the filter element.

In my' Patents Nos. 2,329,194 and 2,323,905, I have disclosed various methods and apparatus for synchronizing and phasing a rotating color filter with respect to a scanning device. Patent No. 2,319,789 to Chambers discloses a similar system in which the color phasing is performed automatically.

In Patent No. 2,323,905 the color lter is driven by an asynchronous motor. A local control wave is generated by the rotation of the filter and its phase compared with that of an incoming synchronizing signal in a phase-comparing circuit. The output of the phase-comparing circuit is supplied to a brake to alter the speed of rotation of the filter so as to maintain it in proper synchronism. Other patents have disclosed dierent means for utilizing a difference in phase between a locally generated wave and an incoming synchronizing wave to effect synchronization. In these systems iield phasing of the rotating iilter has been accomplished by mechanically adjusting the position of the lter on its shaft, or by adjusting the position of the stator of the local generator, or by other mechanical means.

The initial mechanical adjustment may be somewhat troublesome unless provision is made to eilect the adjustment While the receiver is in operation, in which case the necessary mechanical means add to the expense and complication of the receiver, and are likely to get out of adjustment due to vibration, etc. Furthermore, it will be appreciated that if the power line voltage where a receiver is installed differs from that used for the initial adjustment, or changes from time to time, the proper field phase relationship will be disturbed. This results from the fact that a. greater or smaller diierence in phase between the locally generated wave and the incoming synchronizing wave is then required to maintain proper synchronism'. Thus, if the locally generated wave normally lags the incoming synchronizing wave, an increase in the power supply voltage to the asynchronous motor requires a smaller lag in order to maintain synchronism. A similar effect results from an increase in the frictional losses of the rotating parts. Therefore, it is highly advantageous to provide as simple means as possible for adjusting the field phasing ol.' the filter, both for the initial factory adjustment'of the receiver and for servicing the equipment at the point of installation.

It is a primary object of the present invention 3 l to provide an electrical means for adjusting the eld phasing in a simple manner, while the apparatus is inv operation, so as to avoid the expense and complication of mechanical adjustments and permit quick change. Thus an initial adiustment can be readily made at the factory for nominal operating conditions and readiustment made by a service man during the installation of the equipment. It also allows quick change of adiustment to take care of temporary abnormal conditions of operation of the set.

In accordance with the invention, an adjustable electrical phase-shifting circuit is inserted in the circuit of either the synchronizing or locally generated control signals before it is fed to the phase-comparing circuit. The output of the phase-comparing circuit is used to control the speed of rotation of the filter element in any desired manner. Then, if in operation the eld phasing is incorrect, it can be readily adjusted by changing the amount of phase shift introduced by the phase-shifting circuit. The circuit can be madeextremely simple and the adjustment may be a simple screw driver or knob adjustment as desired.

The invention will be more fully understood by reference to the specific embodiment illustrated in the drawings and the following description thereof. In the drawings:

Fig. 1 is a drawing illustrating proper and improper color field phasing;

Fig. 2 is a circuit diagram of an embodiment of the invention;

Fig. 3 is a diagram illustrating theinput and output of the phase-comparing circuit under different conditions of operation:

Fig. 4 is a vector diagram illustrating the manner in which an improper eld phase angle may be corrected; and

Fia'. 5 is an example of an alternative phaseshifting circuit.

Referring now to Fig. l, the rectangle il represents the reproducing area of a receiver tube such as a cathode-ray tube. Arrow indicates the line being scanned at theinstant illustrated. A scanning disk |2 with filters R, G, B rotates in front of the scanning area so as to exhibit successive images reproduced on the scanning area in successively different colors. The segments of filter disk I2 have the configuration described in my Patent No. 2.304.081, but any other type of disk may be employed as desired. The scanning is assumed to proceed from top to bottom, and the disk to rotate counter-clockwise. It will be understood that a filter drum or other form of filter may be employed instead of a disk.

In the position shown in full lines. it will be observed that the red filter is phased slightly in advance of line being scanned, and line ii will be understood to correspond to the red aspect of the image to be reproduced. As scanning proceeds towards the bottom, the red filter will maintain its position slightly in advance so that the lines will be exhibited in the proper color as they are scanned, over the entire image area i0. The width of segment R is sumcient to allow some afterglow of the scanning lines and still exhibit them through the red filter as long as they remain luminous. When the green color-separation image is beginning to be reproduced by scanning line |,"the green lter segment will be in proper l position to exhibit the line therethrough. Thus proper phasing is attained.

While considerable latitude in phasing is available by the design of disk i2, the dotted position shows the disk in an improper phase relationship. If scanning line l I is again assumed to represent a'. red aspect of the image, it will be observed that the line is being exhibited through the blue lter, since the red filter is lagging slightly behind the scanning line. Hence a mixture of blue and red colors will be obtained from line assuming that some afterglow is present. This situation may be corrected by rotating filter disk |2 on its axis through the necessary angle, but this is an inconvenient adjustment to make and requires stopping the apparatus and perhaps making several adjustments before the proper one is obtained.

Referring now to Fig. 2, means are shown whereby the necessary correction in the :field phase angle may be obtained very simply. The color video and accompanying synchronizing signais are received through antenna i5 and supplied to the receiver and synchronizing generator i6. In IS. the video is amplified and detected and supplied to the control grid of cathode-ray receiving tube I1. The synchronizing signals will ordinarily include line and field synchronizing pulsesand are used -to control the generation of suitable horizontal and vertical sawtooth deflecting waves which are applied to the defiecting coils or plates of tube il.

For the system illustrated successive eld scansions correspond to different primary colors. Ii desired, the field synchronizing signal can Vbe used to control the synchronization of the filter disk. Or. distinctive pulses transmitted with each eld of the same color may be used as the filter synchronizing signal to simplify color phasing. The latter signal will be substantially xed in time with the field scansions, even though not of the same frequency.

A sawtooth wave derived from whatever synchronizing signal is chosen is supplied through coupling capacitor I8 and resistor i9 to the grid of a suitable tube, here shown as a triode 2|. Cathode bias for the triode is provided by cathode resistance 22 shunted by the capacitor 23. The late is energized by a suitable source designated In the plate circuit of tube 2| is the primary of transformer 24. The secondary has its midpoint grounded and is shunted by a capacitor 25 which is chosen so as to tune the transformer to the frequency of the sawtooth, thereby converting itv into approximately a sine wave. Since the initial sawtooth voltage applied to the grid of tube 2| is fixed in time with respect to the incoming synchronizing signal, the derived sine wave will also be fixed in time'with respect to the synchronizing signal.

The sine wave so derived is applied to a phaseshifting circuit composed of capacitor 26 and the adjustable resistor 21. This is a conventional phase-shifting circuit whose functioning will be well understood in the art. The output of the phase-shifting circuit is supplied through coupling condenser 28 to the grid of a suitable tube, here shown as a triode 29.

The color lter element, here shown as a disk i2, is driven by a nonsynchronous motor 3| energized from a suitable source designated "mains. On the shaft is an A.C. generator 32 energized from the B+ source and designed to lprovide an output substantially sinusoidal control wave whose frequency varies directly with the speed of rotation of the disk i2 and is the same frequency as that of the incoming synchronizing signal when the filter is rotating at synchronous l animano speed. The output of the generator is supplied through coupling capacitor 33 to the grid of tube 29. Thus both the incoming synchronizing signal, after passing through the phase-shifting network, and the locally generated control signal from generator 32 are supplied to the grid of tube 29. If desired the two signals could be coupled to separate grids of a multigrid tube, thus allowing more gain or separate adjustment of the signal amplitudes.

Resistor 34 is a, coupling resistance and is connected to the variable tap of potentiometer 35 so as to supply a suitable bias for the grid of tube 29. This biasl may be varied to regulate the characteristics of tube 29. In the cathode circuit of tube 29 is a variable resistance 3B which provides any desired amount of degeneration, and is also a means of regulating the characteristics of the tube. The plate is connected through the actuating coil 31 of the D.C. brake 38 to B+.

The operation of a circuit similar to that of Fig. 2, except for the phase-shifting, has been explained in Patent No. 2,323,905 so that only a summary is required here.

Referring to Fig. 3, curve 4D represents the grid voltage-plate current characteristic of tube 29. Wave 39 is assumed to 'be the output of the phase-shifting circuit 26--21 and wave 4I is assumed to be the output of the local generator 32. They are preferably of similar wave form and magnitude, although this is not necessary. ln Fig. 3 they are shown of slightly diierent amplitudes for convenience of identification.

At A the two waves are shown 180 out of phase and only a slight plate current from tube 29 results. Thus only a slight braking action from brake 38 is obtained. Motor 3I will ordinarily be selected so that with this minimum brake current the disk i2 may be brought up to synchronous speed at the lowest mains voltage for which the receiver is designed to operate. For a greater mains voltage the disk i2 will tend to be driven over synchronous speed, thus causing wave 4I to lag wave 39 by less than 180, as shown at B. This will result in a somewhat greater current through tube 29 and hence will increase the brake action. A still greater mains voltage will tend to drive the disk still faster and the waves 39 and 4I may eventually be in phase as shown at C. This results in maximum plate current through tube 29 and maximum braking action.

It willbe apparent that a change in current to the brake can be obtained only by a change in the relative phase of the synchronizing wave 39 and locally generated wave 4I. In the absence of the phase-shifting circuit 26-21, this would also cause a change in the phase of disk I2 with respect to the scanning of the face of cathode-ray tube i1.

Referring now t0 Fig. 4, vector Es represents the phase of the synchronizing signal in synchronizing generator I6. Vector Eph represents the phase of the output of the phase-shifting circuit 26--21 for a given setting, and is shown as lagging Es by an arbitrary amount which takes into account all shifts in phase between the synchronizing generator and the output of the phase-shifting circuit. Vector Eph corresponds to wave 39 in Fig. 3. Vector E1 represents the phase of the local control signal from generator 32 (wave 4I in Fig. 3) and lags Eph by an angle 42 which is necessary to cause the lter disk to rotate synchronously. It is assumed that the position of disk I2 on its shaft, of generator 32 on its shaft, and the adjustment of the phase shifting network 26-21 is such that disk I2 is in proper phase relationship with the vertical scanning.

Therefore, since the phase of the vertical scanning is iixed in time with respect to Es, and the phase of disk I2 is iixed in time with respect to Ei, angle 43 should remain approximately constant for correct eld phasing.

Now assume that due to a decrease in the Voltage of the power mains, or to some other change in the system, the disk tends to rotate slower, so that less brake current is required to maintain synchronization. To produce less brake current, the lag angle of Ei to Eph must increase,

and this is shown by the angle 44 between original vector Eph and new vector 45. However, the angle between Es and vector 45 is greater than angle 43, so that if E1 is allowed to lag to the position of vector 45 the iield phasing will be altered. Hence, resistance 21 in the phase-shifting circuit may be adjusted to decrease the lag angle of Eph with Es, as indicated by vector Eph'. The shift in phase is so chosen as to bring vector 45 back to the position oi vector E1 as indicated by vector E1'. (The slight difference in phase shown is to enable identification.) Phase angle 46 between E1' and Eph is equal to angle 44, so that the brake current remains at the decreased value suilicient to maintain synchronization. Yet the angle between E1 and Es remains at its previous value indicated by angle 43 so that the eld phasing of the disk with the vertical scanning is unchanged.

If the power line voltage should increase, thus decreasing the lag of E1 to Es, resistor 2l can be varied to return E1 to its proper lag angle. It Will thus be apparent that any change in power line voltage or other conditions which would affeet the ileld phasing can be readily corrected by adjustment of the phase-shifting circuit 26-21.

Phase-shifting circuit 26-21 is extremely simple and provides considerable variation in phase angle. A switch can be used in the primary or secondary to extend the range of phase shift if required. lit has the disadvantage that not all the voltage of the secondary of transformer 24 is available at the output of the phase-shifting network, and the output voltage varies somewhat with change of adjustment. Ii these disadvantages are undesirable in any application, other types of phase-shifting networks may be employed, as will be apparent to those skilled in the art. For example, the type shown in Fig. 5 may be employed and has the advantage that the output voltage is equal to the input and electrical phase shifting is possible. This circuit is, of course, somewhat more complicated than the one shown in Fig. 2 but is still comparatively simple.

The phase-comparing circuit including tube 29 is supplied with sinusoidal synchronizing and local control waves to control the operation of the brake. This is not essential and other wave shapes may be employed as desired. Preferably the wave shapes of the two.signals are similar, but even this may be departed from if desired. Furthermore, the sinusoidal synchronizing signal is shown as derived from a sawtooth scanning wave, which in turn is derived from the synchronizing signal in generator IS. Any other method of obtaining the synchronizing wave 39 from the incoming synchronizing signal may be used as desired.

As mentioned before, Fig. 2 may be considered as showing the use of the field synchronizing signal for synchronizing the rotation of the illter aesaceo For example, in a three-color sysi d be synchronized with said scanning device and the relative phasevtherebetween varied.

3. In a color television system utilizing a sequential color video signal with a synchronizing signal in fixed time relationship therewith,

,. apparatus comprising means for reproducing 2i or may be converted into a sine wave, or

other wave form, in any other desired manner.

It will be understood that such color synchronizing pulses will be substantially synchronized with the field scansions even though they do not recur at the same frequency.

Instead of inserting the phase-shifting circuit in the path of the incoming synchronizing signal, it may be inserted in the path of the local control signal between generator 32 and tube 29.v

The functioning of such a modification will be clear to those in the art.

The preceding paragraphs have mentioned a number of variations in the specic embodiment described. The phase-shifting circuit may also 4be employed at the transmitter, as mentioned hereinbefore, to maintain the transmitter color lter in proper synchronism and phase with respect to the scanning at the transmitter tube. Many other variations will occur to those in the art within the scope of the invention as dei-ined in the claims.

I claim: 1. In a color television system of the sequential comprising a scanning device substantially inV synchronism with said synchronizing signal, Va movable color filter element cooperating with said scanning device, a nonsynchronous motor coupled to drive said color filter element, a generator coupled to said color filter element and adapted to produce a control signal, control means responsive to a change in phase of two signals supplied thereto to change the speed of movement of said color filter element, an adjustable phase-shifting circuit, and means for supplying one of said synchronizing and control signals to said control means and the other through said adjustable phase-shifting circuit to said controlk means, whereby the color filter element may be synchronized with said scanning device and the relative phase therebetween electrically adjusted.

2. In a color television system of the sequential type having a synchronizing signal in fixed time relationship with the video signal, apparatus comprising a scanning device in synchronism with said synchronizing signal, a rotatable color filter element` cooperating with said scanning device, a nonsynchronous motor coupled to drive said color filter element, a generator coupled to said color lter element and adapted to generate a control signal whose frequency varies with the speed of rotation thereof, control means responsive to a change in phase of two signals supplied thereto to change the speed of rotation of said color filter element, a variable electrical phaseshifting circuit, and means for supplying one of said synchronizing'and control signals to said control means and the other through said variable electrical phase-shifting circuit to the control means, whereby the color iilter element may successive color-separation images from said color video signal, a movable color filter elementfor presenting successive color-separation images in their respective colors, a nonsynchronous motor coupled to drive saldcolor filter element, a iocal generator coupled to' said movable color filter element and adapted to produce a local control signal, control means responsive to a change in phase of two signals supplied thereto to change the speed of movement of said color lter element, an adjustable phase-shifting circuit, and means for supplying one of said synchronizing andlocal control signals to said control means and the other through saidadjustable phaseshifting circuit to said control means, whereby the color ltereieiuent may be synchronized with the reproduction of successive color-separation images and the relative phase therebetween electrically adjusted.

4. In a color television system utilizing a sequential color video signal with a synchronizing signal in fixed time relationship therewith, apparatus comprising means for reproducing successive color-separation images from said color video signal substantially in synchronism with said synchronizing signal, a rotatable color lter element for presenting successive color-separation images in their respective colors, a nonsynchronous motor coupled to drive said color filter element, av local generator coupled to said rotatable color filter element and adapted to produce a local control signal whose frequency varies with the speed of rot ation thereof, control means responsive to a charge in phase of two signals supplied thereto to change the speed of rotation of said color filter element, a variable electrical phase-shifting circuit, and means for supplying one of said synchronizing and control signals to said control means and the other through said adjustable phase-shifting circuit to said control means, whereby the color lter element may be synchronized with the reproduction Yof ,successive color-separation images and the relative phase therebetween electrically adjusted.

5. In a color television system for receiving a sequential color video signal with a synchronizing signal associated therewith, apparatus comprising means for reproducing successive colorseparation images from said color video signal substantially in synchronism with said synchronizing signal, a rotatable color` filter element for presenting successive color-separation images in their respective colors, a nonsynchronous motor coupled to drive said color lter element, a local generator coupled to said rotatable color lter element and adapted to produce a local control signal whose frequency varies with the speed of rotation thereof, means for converting said synchronizing signal into a wave form substantially similar to that of said local control signal, control means responsive to a change in phase of two signals supplied thereto to change the speed of rotationof said color iilter element, a variable electrical 'phase-shifting circuit, and means for supplying one of said converted synchronizing and local control signals to said control means and the other through said adjustable phaseshifting circuit to said controlmeans, whereby the color filter element may be synchronized 9 with the reproduction of successive color-separation images and the relative phase therebetween electrically adjusted.

6. In a color television system for receiving a sequential color video signal with a synchronizing signal associated therewith, apparatus comprising receiver circuits and associated cathode-ray tube for reproducing successive colorseparation images from said color video signal substantially in synchronism with said synchronizing signal, a rotatable color filter element associated with said cathode-ray tube for presenting successive color-separation images in their respective colors, a non-synchronous motor coupled to drive said color filter element, a local generator coupled to said color lter element and adapted to generate a local control signal of the frequency of said synchronizing signal when the filter element rotates `at synchronous speed, means for converting said synchronizing signal into a Wave form substantially similar to that of said local control signal, control means for producing an output which changes in response to a change in phase difference of two signals applied thereto, a variable electrical phase-shifting circuit, means for supplying one of said converted synchronizing and local control signals to said control means and the other through said variable phase-shifting circuit to said control means, and means for controlling the speed of rotation of the color iilter element in accordance with the output of the control means, whereby the color illter element may be synchronized with the reproduction of successive color-separation images and the relative phase therebetween electrically adjusted.

'7. In a color television system for receiving a sequential color video signal with a synchronizing signal associated therewith, apparatus comprising receiver circuits and associated cathoderay tube for reproducing successive color-separation images from said color video signal substantially in synchronism with said synchronizing signal, a rotatable color filter element associated with said cathode-ray tube for presenting successive color-separation images in their respective colors, a nonsynchronous motor coupled to said color ilter element and tending to drive the element over synchronous speed, a local generator coupled to said color filter element and adapted to generate a local control signal of the frequency of said synchronizing signal when the iilter element rotates at synchronous speed, means for converting said synchronizing signal into a wave form substantially similar to that o! said local control signal, control means for producing an output which changes in response to a change in phase diiierence of two signals applied thereto, a variable electrical phase-shitting circuit,

l means for supplying one of said converted synchronizing and local control signals to said control means and the other through said variable phase-shifting circuit to said control means, a brake mechanism adapted to brake the rotation of the color filter element, and means for controlling the operation or the brake mechanism in accordance with the output of the control means,

whereby the color lter element may be synchronized with the reproduction of successive colorseparaton images and the relative phase therebetween electrically adjusted.

8. Apparatus for synchronizing a movable element with a synchronizing signal and adjusting the phase therebetween which comprises a nonsynchronous motor coupled to drive said movable element, a local generator coupled to said movable element and adapted to produce a local control signal, an adjustable phase-shifting circuit supplied with one of said synchronizing and local control signals, control means responsive to a change in phase of two signals supplied thereto to change the speed of movement of said movable element, and means for supplying said one signal from the adjustable phase-shifting circuit and the other of said synchronizing and local control signals to said control means, whereby the movable element may be synchronized with said synchronizing signal and the relative phase therebetween adjusted.

9. Apparatus for synchronizing a rotatable element with a synchronizing signal and adjusting the phase therebetween which comprises a nonsynchronous motor coupled to drive said rotatable element, a local generator coupled to said rotatable element and adapted to produce a local control signal whose frequency varies with the speed oi rotation of the element, a variable electrical phase-shifting circuit supplied with one oi said synchronizing and local control signals. control means for producing an output which changes in value in response to a change in the relative phase of two signals supplied thereto, means for supplying said one signal from the variable electrical phase-shifting circuit and the other of said synchronizing and local control signals to said control means, and means for controlling the speed of rotation of the rotatable element in accordance with the output of said control means.

PETER C. GOLDMARK.

REFERENCES CITED The following references are of record in the ille of this patent:

UNITED STATES PATENTS Number Name Date 2,323,904 Goldmark July 13, 1943 

